The invention provides a symmetric fully-homomorphic encryption method based on a plaintext similarity matrix. The symmetric fully-homomorphic encryption method aims to solve the technical problem of low efficiency of symmetric fully-homomorphic encryption at present, and is implemented by the steps that: a user generates two big prime numbers with the same length according to a requirement, constructs a residue class ring according to the generated big prime numbers, constructing a general linear group according to the residue class ring, calculates a homomorphic calculation public key and a symmetric secret key, encrypting a similarity matrix of plaintext matrixes by using the symmetric secret key, and decrypting ciphertext matrixes by using the symmetric secret key; a cloud server uses the homomorphic calculation public key for conducting homomorphic calculation on the ciphertext matrixes; and the user uses the symmetric secret key for decrypting a homomorphic ciphertext matrix. The symmetric fully-homomorphic encryption method is simple in the secret key selection and encryption process, hides the plaintext matrixes randomly, improves the safety of an encryption algorithm, does not introduce noise in the ciphertext calculation process, can conduct arbitrary calculation on the ciphertext matrixes according to needs, and can be applied to full-course encryption state protection of important data in cloud computing, big data environments and the like.